The present invention relates to a storage device and a method for managing the storage device.
In a flash memory, each level of a threshold voltage (Vth) of a transistor is set in correspondence with data such as 0 or 1 and so forth. In addition, in the flash memory, desirable data is stored by injecting electric charges into a trap layer (also called a floating gate) which is installed in a gate part of the transistor and thereby manipulating the threshold voltage. However, the amount of electric charges stored in the trap layer is decreased at various speeds by factors such as passage of time, an environment, a rewriting frequency and so forth. Accordingly, it is difficult to grasp the current amount of electric charges and there is a risk that stored data may be erroneously read out.
Therefore, a technology of managing a data holding period (also called a retention period) is proposed. For example, Japanese Unexamined Patent Application Publication No. 2000-11670 discloses equipment which has a timer for measuring a time elapsed after a writing operation has been executed on a nonvolatile memory and a refresh circuit which executes the writing operation again on the nonvolatile memory when the elapsed time which is measured by the timer has exceeded a predetermined time.